[1]} proposed the first known sequential test procedure in which a decision to stop or continue collecting data was based on prior data, recognizing that decisions to stop or continue a trial made based on prior observations could substantially reduce the expected numbers of required subjects; it was a two-stage design. [2]} devised a multiple sequential testing procedure for binomial data based on [3]}'s likelihood ratio test that [4]} cites as a "forerunner" to his more general sequential probability ratio test (SPRT) procedure in which the probabilities of type I and II errors are controlled. Extensions with stopping decisions based on groups of subjects [Group Sequential Designs (GSDs)] are given in [5]}.
